2198
J. Phys. Chem. C 2007, 111, 2198-2204
Reduction of an r-Fe2O3(0001) Film Using Atomic Hydrogen Weixin Huang,*,† Wolfgang Ranke,‡ and Robert Schlo1 gl‡ Department of Chemical Physics and Hefei National Laboratory for Physical Sciences at the Microscale, UniVersity of Science and Technology of China, Hefei 230026, China, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany ReceiVed: October 6, 2006; In Final Form: NoVember 24, 2006
The R-Fe2O3(0001) biphase surface consists of an ordered arrangement of FeO(111) and R-Fe2O3(0001) surface domains on a Fe2O3 bulk [N.G. Condon et al., Phys. ReV. Lett. 75, 1961, 1995]. The interaction of atomic hydrogen Hat with this surface has been investigated at room temperature (RT, 300 K) by means of lowenergy electron diffraction, X-ray photoelectron spectroscopy, and thermal desorption mass spectroscopy. The surface is easily hydroxylated by Hat. Upon heating, the OH groups react to produce hydrogen and water, the latter of which results in the partial reduction of the surface. In a parallel but slower process, bulk reduction proceeds already during exposure at RT. First, Fe3O4(111) domains, embedded in the R-Fe2O3(0001) matrix, are formed. Finally, the film is completely reduced to Fe3O4. Further reduction toward FeO or metallic Fe appears kinetically hindered and is not observed under the low exposures used in our experiments.
1. Introduction Metal oxides are widely employed in heterogeneous catalysis, not only as supports but also as catalysts. Understanding and controlling oxide surfaces are the key issues for the development of industrial oxide catalysts and related advanced materials.1 Oxide surfaces are, however, in general heterogeneous and complicated. Therefore in order to reduce the level of complexity, a common approach is to study model catalysts such as single-crystal oxide surfaces or flat surfaces of epitaxial oxide thin films employing surface science techniques.2-5 Here in order to adequately approach practical catalytic systems, a key issue is to prepare each kind of surface species on the model catalyst which is observed on the corresponding practical catalyst. Sometimes this is a tough task because some activation processes of gas molecules easily occurring on surfaces of realistic oxide catalysts under practical catalytic reaction conditions may not take place on surfaces of model oxide catalysts under ultrahigh vacuum (UHV) conditions. Molecular hydrogen adsorption is an example. Although the presence of hydrogen on a metal oxide surface is common and has a pronounced influence on its chemical and electrical properties,6,7 detailed studies about the formation of hydrogen adlayers on model oxide surfaces are rather scarce.2,8,9 The main reason is the extremely low dissociative sticking coefficient of molecular hydrogen. Recently, Wo¨ll et al. reported a series of elaborated surface science studies on the adsorption of atomic hydrogen on various model surfaces of ZnO,10-13 deepening the fundamental understanding of physicochemical properties of zinc oxide induced by hydrogen adsorption. Iron oxide based catalysts are employed in the important industrial catalytic process of dehydrogenation of ethylbenzene to styrene.14 A great deal of insight into this reaction has been acquired by studying the surface chemistry and catalytic performances under high-pressure reaction condition of various * To whom correspondence should be addressed. E-mail: huangwx@ ustc.edu.cn. Fax: +86-551-3601592. † University of Science and Technology of China. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft.
well-defined epitaxial iron oxide films (FeO(111), Fe3O4(111), and R-Fe2O3(0001)) grown on Pt(111). An R-Fe2O3(0001) film was found to have a high catalytic activity, which was attributed to both the presence of Fe3+ and its intermediate adsorption strength for ethylbenzene and styrene.15-24 In order to be catalytically active, the surface needs to be defective, but the nature of the defects is still unknown.25 Deactivation occurs by reduction to Fe3O4 and by coking. Both can be effectively suppressed by addition of small amounts of oxygen to the reactants, which is a mixture of ethylbenzene and steam in excess.21,22 Some reaction steps in the catalytic cycle are not yet clear such as the mechanism of hydrogen removal from the catalyst after the dehydrogenation step of ethylbenzene. The observed reduction of Fe2O3 suggests that hydrogen forms OH groups which recombine and desorb as H2O. In the steady state, however, a reoxidation is required, possibly by dissociation of water from the feed. An alternative to this so-called Mars-van Krevelen mechanism is direct desorption of molecular H2.26 A reasonable surface science approach to this issue is to study the reactivity of surface hydroxyl groups on model iron oxide catalysts. However, due to the inertness of the model oxide surface, dissociative adsorption of molecular hydrogen cannot effectively generate surface hydroxyls. Employing atomic hydrogen, we have successfully prepared surface hydroxyls on model iron oxide surfaces, studied their reactivity and found that they react to produce both hydrogen and water.27,28 In the present paper, we report in detail on the interaction of atomic hydrogen with an R-Fe2O3(0001) biphase surface consisting of a long-range ordered array of FeO(111) and R-Fe2O3(0001) islands.29 2. Experimental Section The experiments were performed in a stainless steel UHV chamber with a base pressure of 1 × 10-10 mbar. The UHV chamber was equipped with a back-view low-energy electron diffraction (LEED) optics (Omicron), a hemispherical electron energy analyzer (Phoibos 150, Specs), a dual-anode X-ray source (VSW, used line Al KR) for X-ray photoelectron spectroscopy
10.1021/jp066584u CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
Reduction of Fe2O3(0001) (XPS), and a quadrupole mass spectrometer (QMS, Balzers Prisma) for TPD. The QMS was equipped with an entrance cone (3 mm diameter) so that only gas from the sample center could enter the QMS in line of sight. A Pt(111) sample (9 mm diameter, 1.5 mm thick) with a type K thermocouple spot welded to its edge was mounted on a sapphire support which could be introduced into the chamber via a transfer system. The epitaxial oxide film was prepared on the clean Pt(111) substrate by evaporating iron from an Fe wire wrapped around a heated W filament, followed by a proper oxidation cycle in 10-6 mbar O2.20 A high-efficiency source for atomic hydrogen (Hat) was constructed following the ideas of Bischler and Bertel.30 The hydrogen gas passes through a W capillary the tip of which is heated by electron bombardment to about 2000 K resulting in a high degree of dissociation. The capillary is surrounded by a copper heat shield. At the sample position 4 cm in front of the capillary, the diameter of the Hat beam is estimated to be large enough to yield a fairly homogeneous exposure. Given exposure values (in Langmuir units, 1 L ) 1.33 × 10-6 mbar‚s) refer to the H2 background pressure and are relative values concerning Hat. The actual exposures to Hat are unknown, but the observed saturation of hydroxyl coverage after only 1 L (see below) suggests that Hat exposures must have been of the same order of magnitude as the measured H2 exposure. Exposure was performed at room temperature (RT). During thermal desorption experiments, a linear heating rate of 4.3 K s-1 was employed. The sample was cooled to 230 K prior to starting of TDS experiments. In this way, the main part of the unavoidable desorption of H2 from the heater (filament and its surrounding) will occur when the sample temperature is far below 300 K and thus could be separated from the desorption process from the sample. Only a structureless sloping background remained in TDS spectra, which could be separated from the sample signal. After each experiment, the surface was restored by annealing at 900 K in 10-6 mbar oxygen for several minutes. Restoration was examined by XPS and LEED.
J. Phys. Chem. C, Vol. 111, No. 5, 2007 2199
Figure 1. LEED patterns of: (a) clean Pt(111); (b) the surface after one cycle of iron deposition and oxidation; (c) after two cycles of iron deposition and oxidation representing Fe3O4(111); (d) the final clean R-Fe2O3(0001) biphase surface. The solid and dashed lines indicate the reciprocal unit cells for FeO(111) 1 × 1 and R-Fe2O3(0001) 1 × 1, respectively. EP ) 60 eV.
3. Results 3.1. Growth and Characterization of An R-Fe2O3(0001) Biphase Thin Film. The epitaxial growth and LEED patterns of various iron oxide films on Pt(111) has been well established.20 A 1-2 monolayer (ML) thick FeO(111) film and a thicker (3-20 nm) Fe3O4(111) film can be prepared by cycles of iron deposition and oxidation in 10-6 mbar oxygen at 9001000 K, whereas an R-Fe2O3(0001) film with uniform termination can only be acquired by oxidizing an Fe3O4(111) film at pO2 > 10-2 mbar at 970-1100 K. In our experiments, we prepared the oxide film by repeated cycles of iron evaporation and oxidation at 850 K in 10-6 mbar oxygen followed by a final annealing at 900 K in 10-6 mbar oxygen. As expected from a phase diagram of the Fe-O2 system,31 Fe2O3 is formed but the surface termination is “biphase”. Figure 1 presents LEED patterns recorded during the preparation of the iron oxide film. The final surface shows a composite LEED pattern that can be considered as a superposition of two symmetries: one arising from R-Fe2O3(0001) 1 × 1 (dashed line in Figure 1d), the other from FeO(111) 1 × 1 (solid line in Figure 1d). But each spot arising from R-Fe2O3(0001) 1 × 1 is surrounded by narrow-spaced satellite spots. The same LEED pattern has been observed on an R-Fe2O3(0001) single-crystal treated by sputtering followed by oxidation in 10-6 mbar oxygen and was attributed to a surface reconstruction.29,32 STM observation revealed that the surface with such a LEED pattern
Figure 2. Fe 2p and O 1s XPS spectra of the clean R-Fe2O3(0001) biphase film. The inset shows the Pt 4f XPS spectra before and after the growth of the R-Fe2O3(0001) biphase film.
is stabilized by the coexistence of mesoscopic islands revealing lattice periodicities of R-Fe2O3(0001) and FeO(111), with the islands themselves arranged to form a superlattice.29 Figure 2 shows the XPS spectra of the prepared film. The peak maxima (binding energies) of Fe 2p3/2 and 2p1/2, respectively, lie at -711.2 and -724.6 eV, that of O1s at -530.2 eV, which is consistent with peak positions of a thick R-Fe2O3(0001) film on Pt(111).33 Very clearly, a peak appears at 719.1 eV, which is a characteristic shakeup satellite line for Fe3+ species. The bulk of the film is thus Fe2O3, but the surface consists of a well-ordered arrangement of FeO(111) and R-Fe2O3(0001) domains. Such an R-Fe2O3(0001) surface is denoted R-Fe2O3(0001) biphase. Using a Lambert-Beer-like attenuation law I/I0 ) exp(-d/λ) with an electron mean free path λe ) 2 nm for the Pt 4f7/2 peak intensity (Ekin ≈ 1410 eV), the film thickness d turns out to be 3.3 nm. With consideration of the involved error, it is thus estimated to be 3-4 nm. The LEED and XPS data reveal that the bulk of an Fe3O4(111) film can be fully oxidized into R-Fe2O3(0001) in 10-6 mbar O2 at 900 K, in full agreement with the Fe-O2 phase
2200 J. Phys. Chem. C, Vol. 111, No. 5, 2007
Huang et al.
Figure 4. TDS traces of water and hydrogen of two consecutive TDScycles after 1000 L Hat at RT without reoxidizing the surface in 10-6 mbar oxygen in between.
Figure 3. TDS spectra of water and hydrogen after various exposures of Hat on the clean R-Fe2O3(0001) biphase surface at RT.
diagram,31 while the surface remains in a mixed oxidation state. In order to obtain a uniform R-Fe2O3(0001) surface termination, oxidation has to be performed at higher oxygen pressure (above 10-2 mbar oxygen at 970-1100 K).20 3.2. Adsorption of Had on R-Fe2O3(0001) Biphase. Figure 3 displays the TD spectra of the only desorbing species, water and hydrogen, from the surface after various exposures to Hat at RT. Dependent on the Hat exposure, the H2 desorption traces start from an increased background level, which, as explained in the experimental section, is due to the unavoidable desorption of H2 from the heater (filament and its surrounding). After 1 L Hat, a water desorption peak arises at 348 K, but no hydrogen desorption is observed. Increasing the Hat exposure to 10 L shifts the water desorption peak down to 327 K and a weak hydrogen desorption structure appears to overlap the background at ∼330 K. The water desorption trace changes unexpectedly after a dose of 100 L Hat. A peak appears at ∼340 K, but its intensity has decreased. Two weak H2 desorption peaks at 303 K and the other at 400 K appear to overlap the sloping background. A large dose of 1000 L Hat is necessary to develop clear and intense desorption structures for water (main peak at 375 K, small peak at ∼445 K, and a very weak structure at ∼480 K) and hydrogen (415 K). When a second TDS cycle with 1000 L Hat was performed without reoxidizing the surface (Figure 4), the water desorption around 445 K intensifies at the expense of the peak at 375 K. As discussed later, we assign this to the increasing reduction of the near-surface region. Figure 5a shows the XP spectra of Fe 2p of the clean surface, the surfaces after Hat exposure at RT, and the surfaces after Hat exposures and heating to 780 K, corresponding to the state after TDS. Exposure induces an increase in the low binding energy tail which indicates a partial reduction of the surface.
The extent of reduction increases with the increasing Hat exposure. Heating the exposed surface to 780 K restores the surface exposed to 1 L of Hat almost completely. We conclude that reduction was confined to a thin surface layer which was restored during heating by Fe or O exchange with the bulk. Upon 1000 L Hat, annealing cannot restore the clean surface spectrum. Reduction extends too deep into the bulk, probably the whole film (thickness ∼3-4 nm) is reduced. The other exposures represent intermediate steps. The labels marked “PP” give the peak positions for different oxide phases and metallic iron as observed in a previous study.33 After exposure to 1000 L Hat, the peak position has only very slightly shifted to a position compatible with an average reduction to Fe3O4. This is confirmed by the analysis of the extrapolation of the steep and almost linear part of the low-BE edge toward the baseline (edge extrapolation value, EE, also derived from the data in ref 34). In a mixture of phases, this extrapolation would always be dominated by the position for the most strongly reduced component. The edge extrapolation is also compatible with Fe3O4. FeO or metallic Fe phases are not formed or represent only a small minority. Fe3O4 formation is further supported by the changes in the satellite region. Fe2O3 shows a typical, Fe3+related satellite peak at -719.1 eV (a detailed analysis of peak shapes and satellite shifts is found in refs 35 and 36). Reduction causes smearing of the spectra in this region in a way typical for Fe3O4.33,35,36 The Fe3+ satellite is reduced in intensity according to the decrease of Fe3+ in Fe3O4, the minimum left to it is filled by the reduction-induced broadening of the Fe 2p1/2 peak, and the minimum right to it is filled by the newly formed Fe2+ satellite contribution centered at 715.5 eV. The Fe 2p spectra thus tell us that Hat at RT easily induces reduction to Fe3O4. Further reduction must be considerably more slowly and does not occur for the applied exposures at RT. The reduction comprises the whole film of 3-4 nm thickness which proves that Fe or O is sufficiently mobile even at RT. O 1s spectra for the same sample treatments are shown in Figure 5b. Exposure causes a tail toward higher BE. Similar to the Fe 2p spectra, annealing can restore the 1 L spectrum but not the 1000 L spectrum. Again, the other exposures represent intermediate steps. The O 1s spectrum of the clean surface has a slightly asymmetric peak shape whose reasons go beyond the scope of this paper. In order to decompose the spectra after adsorption, we have therefore fitted them using the peak shape of the measured O 1s spectrum of the clean surface as the component shape. Results for 1L and 1000 L Hat are shown in Figure 6. A
Reduction of Fe2O3(0001)
J. Phys. Chem. C, Vol. 111, No. 5, 2007 2201
Figure 5. Fe 2p (a) and O 1s (b) XPS traces of clean surfaces (solid line, black), surfaces after Hat exposures at RT (dashed, red), and surfaces after Hat exposures at RT followed by heating to 780 K (dotted, green). The labels PP and EE in (a) mark the peak maximum positions and the edge extrapolation positions, respectively, for (from left to right) R-Fe2O3, Fe3O4, FeO, and Fe.
TABLE 1: Contributions from Different Oxygen Components to the O 1s XPS Spectra Following Hat Exposure at RT on Basis of the Peak-fitting Results Hat exposure 1L 10 L 100 L 1000 L
Figure 6. Peak-fitting results of O 1s XPS spectra following exposures of 1 and 1000 L Hat at RT: circles represent original data and lines represent fitted data.
fit with two components of equal shape and width is not satisfying, but a fit with two components of different width works well for 1 L. The main component at -530.2 eV corresponds to the lattice oxygen of the iron oxide; the second component at -532.0 eV is ascribed to hydroxyl groups. The width of this component is ∼1.3 times that of the main component.
M
C3
OH
94.4% 92.8% 91.4% 87.4%
0% 1.6% 3.0% 7%
5.6% 5.6% 5.6% 10%
A fit with only two components for higher exposures yields ∆E values unreasonably small and decreasing with exposure. The widths of the shifted component get unreasonably large. It seems either that a third component appears between the unshifted and the OH-derived component or that the main component changes its shape, i.e., gets broader and more asymmetric. This is supported by the spectra after flashing. After 1 L, flashing can almost restore the clean surface spectrum. After 100 L and especially 1000 L H, it cannot (see Figure 5b). The remaining broadened spectrum consists either of two components or of one with changed shape. Formally, we have performed fitting for higher exposures using three components (see Figure 6b for 1000 L Hat) with the following restrictions: The OH-component is fixed at a ∆E ) 1.8 eV (as in the 1 L spectrum). The width is free for fitting. The third component C3 has a fixed width equal to that of the main component. Its position is free for fitting. The resulting fits are very good, and the widths of the OH components turn out quite similar for all exposures which is reasonable. Table 1 lists the relative intensities of the three components for different exposures. The position of the O 1s peak maximum for different iron oxide phases has been reported to be equal.33,34 But, to our knowledge, a detailed analysis of peak shape differences has not been published. Therefore, we are not sure if the component C3 really corresponds to a third oxygen species. It could, of course, be related to defects that are likely to form upon reduction at RT. In this case it should disappear or at least be reduced upon heating which, as mentioned, does not occur. Therefore, we tentatively ascribe the sum of the main component M and the component C3 to the oxide, which increasingly gets reduced with increasing exposure and which according to the
2202 J. Phys. Chem. C, Vol. 111, No. 5, 2007
Huang et al.
Figure 7. Sections of LEED patterns corresponding to the lower right part of patterns in Figure 1 for the clean R-Fe2O3(0001) biphase surface and after different exposures to Hat at RT and after flash to 780 K. The spots indicated by arrows arise from Fe3O4(111) domains. Ep ) 60 eV.
Fe 2p peak analysis is Fe3O4. In fact, when comparing the O 1s spectra of clean Fe2O3 with those of Fe3O4,28 it turns out that the peak from Fe3O4 is broader and more asymmetric. According to an estimation, based on average electron mean free paths28 using λe ) 1.67 nm for the O 1s electrons with Ekin ≈ 950 eV), the saturation intensity of the OH peak for 1-100 L Hat (see Table 1) corresponds to about 1/3 of a densely packed oxygen layer in Fe2O3 and thus to about one OH per surface unit cell. The change of geometrical structure of the R-Fe2O3(0001) biphase surface induced by Hat adsorption was monitored by LEED. The results are shown in Figure 7. Exposing the surface to 1L Hat at RT decreases the spot intensity in the LEED pattern only slightly, but further increasing the Hat exposure removes the satellite spot pattern completely, which indicates that the long-range order of the R-Fe2O3(0001) biphase surface is destroyed. Among the remaining spots, those corresponding to the FeO periodicity seem to be weakened faster than the others, which might be an indication that the FeO domains are more reactive and are more quickly reduced than the Fe2O3 domains of the biphase structure. A fast partial reduction of a thin FeO film has been observed before.28 However, there is not any indication from the XPS measurements that a reduction state lower than corresponding to Fe3O4 (mixed Fe2+ and Fe3+) is formed at a measurable level. Therefore, we ascribe the preferential weakening of FeO spots rather to a faster disordering of the FeO domains. Even without heating, weak new spots corresponding to Fe3O4(111) are formed after 1000 L Hat. Heating the Hat-exposed surface to 780 K restores the satellite spot pattern almost completely for low exposure and partially for high exposures. Already after 10 L Hat and annealing and very clearly after 100 and 1000 L Hat and annealing, additional spots arising from Fe3O4(111) domains20 appear in the LEED pattern. For a detailed analysis of the complex LEED pattern, Figure 8 shows LEED patterns, clean and after 1000L Hat, flash and reoxidation in 10-6 mbar O2 at 900 K. The upper and the lower row are the same images, but in the lower row explanations are superimposed. The satellite pattern of the clean sample in Figure 8A is fully explainable by the Fe2O3-FeO biphase structure (dashed grid). After 1000 L H (B), the satellite spots have completely disappeared, which means that there exists no more long-range order of the biphase arrangement. Simultaneously, weak Fe3O4 spots appear (dark dashed grid lines), most clearly seen is the
Figure 8. Sections of LEED patterns corresponding to the lower right part of patterns in Figure 1 for the clean surface, after exposures to 1000 L Hat at RT, after flash to 780 K and after reoxidation (10-6 mbar O2, 900 K). Upper and lower patterns are the same. In the lower row, the narrow-spaced grid of the Fe2O3(0001) biphase satellites and the grid of Fe3O4 (dark dashed lines) are added. In (C), the circles mark the spot position for the indicated oxide phases. For the spots marked by circles in (D) see text. The spots indicated by arrows arise from Fe3O4(111) domains. Ep ) 60 eV.
spot in the upper right region of the image (arrow). After flashing, Figure 8C, the long-range order is improved. The Fe3O4 spots get more intense and the satellite pattern reappears partially. Obviously we have a mixture of Fe2O3 and Fe3O4 domains. The lower right Fe3O4 spot overlaps with the spots from the other oxide phases and fills partially the minimum between the FeO and the Fe2O3 spot. This contributes to blurring of the pattern. Reoxidation for 35 min, Figure 8D, cannot reestablish the Fe2O3 completely as evidenced by the weak but still existing Fe3O4 spots. But the order has improved and the satellite pattern is clearer. When comparing it to that of the initial clean surface Figure 8A, one can see that intensity or spots have appeared at some positions where no or only weak intensity was seen in the clean pattern (highlighted by white circles). Their positions do not fit with those expected for the Fe2O3FeO biphase structure but would be explainable by a Fe3O4 structure similar to that observed and analyzed by Berdunov et al.37 These authors get this structure when they “overoxidize” the Fe3O4 (10-6 mbar O2, 950 K, cool down in O2) under conditions very similar to our reoxidation conditions (10-6 mbar O2, 900 K, cool down in O2 to below 600 K). We conclude that a low exposure of 1 L Hat leaves the structure almost unaffected. The tail observed in the Fe 2p spectrum is due to the reductive influence of OH at the surface but no observable amounts of O vacancies are formed. In agreement with XPS, flashing can reestablish the surface order completely. Higher exposures cause increasing in-depth reduction as evidenced by the appearance of Fe3O4 spots. Again in agreement with XPS, flashing cannot re-establish the Fe2O3 biphase structure completely. An appreciable part of the surface orders as Fe3O4 with an own satellite structure in LEED The overlap of the patterns from Fe2O3 and Fe3O4 indicates phase separation. LEED is not suitable to quantify the portions of both terminations and it cannot give information about the phases in the bulk. That the Fe3O4 phase is not even removed after prolonged reoxidation of the surface after exposure to 1000 L
Reduction of Fe2O3(0001)
J. Phys. Chem. C, Vol. 111, No. 5, 2007 2203
Hat (Figure 8D) suggests that in-depth reduction had been strong which again agrees with XPS. Up to 100 L, the intensity of the OH component is constantly 5.6%, which therefore represents a saturation coverage. Already at 1 L, the OH coverage has thus saturated. The component C3 is zero which shows that in-depth etching does not yet occur which is in agreement with the full restoration of both the Fe 2p and the O 1s spectra after annealing. 4. Discussion Exposure of the hematite R-Fe2O3(0001) biphase surface to atomic Hat at RT results in surface hydroxilation for all used exposures (1 - 1000 L). The OH-induced contribution in the O 1s spectra indicates a saturation value for 1-100 L, which corresponds to a coverage of about one OH per surface unit cell of Fe2O3. This result also proves that the surface was really hydrogen-free before exposure. After 1000 L, the OH coverage has increased which might be due to increased disorder and the admixture of Fe3O4 domains. Besides surface hydroxilation, an exposure-dependent reduction of the bulk of the film occurs during RT exposure. Both XPS and LEED show that bulk reduction is negligible after 1 L Hat. Heating to 780 K restores the surface structure completely. After 1000 L Hat, however, the shape of the Fe 2p spectrum has changed to that of magnetite, Fe3O4, and heating cannot restore its peak shape which shows that the oxygen reservoir of the bulk is fairly exhausted. This demonstrates that Fe or O are sufficiently mobile even at RT to equilibrate over distances of the film thickness (3-4 nm). It demonstrates further the advantage of using thin films where the limitation of the bulk reservoir allows to observe such an effect clearly. Also the long-range order of the film is affected by the etching effect of Hat. While 1 L Hat affects the order only marginally, 10 L are sufficient to destroy the satellite pattern in LEED which corresponds to the long-range biphase ordering of FeO and Fe3O4 surface domains. However, the LEED spots corresponding to the Fe2O3 bulk periodicity are conserved, which means that the short-range order remains intact. And even new spots from the reduced Fe3O4 phase appear after 1000 L Hat. Heating re-establishes the satellites of the Fe2O3 biphase surface and enhances the Fe3O4 spots. Even after a fairly complete reduction of the bulk to Fe3O4 (after 1000 L Hat), the Fe2O3 biphase structure occupies an appreciable part of the surface indicating that it represents a quite stable surface termination. In a second 1000 L Hat exposure-TDS cycle (Figure 4), the water desorption peak at 445 K grows at the expense of the desorption peak at 375 K. Also at 580 K, a small peak appears to develop. With LEED, the formation of Fe3O4(111) islands on the surface was observed after the first cycle. Therefore, we assign the water desorption peak at 445 K and the structure at 580 K to recombination of surface hydroxyls on Fe3O4(111) islands. The positions agree fairly well with those for surface hydroxyl recombination on a thick Fe3O4(111) film, where peaks were found at 440 and 560 K.28 Accordingly, the water desorption peak at 375 K and the hydrogen desorption peak at 415 K are ascribed hydroxyl recombination on the Fe2O3 biphase surface on the reduced bulk. A key for the partial reduction of iron oxides at RT by Hat is that the resulting product, water, can desorb from the surface at or below RT. For a FeO(111) thin film the water desorption maximum in TDS was found at 170 K, for a thick Fe2O3(0001) film it is at 260 K and for thick Fe3O4 at 280 K.20 The reduction rate is therefore supposed to be determined mainly by bulk
Figure 9. Possible reduction mechanisms, starting from a partially hydroxylated surface, see text.
diffusion of Fe (from the surface to the bulk) or O (from the bulk to the surface). Since we know the thickness of our Fe2O3 starting film (3-4 nm), we can estimate the amount of H2O being released during reduction to Fe3O4. It amounts to about five H2O molecules per Fe2O3 surface unit cell (0.109 nm2, corresponding to the dashed reciprocal unit cell in the LEED pattern, Figure 1d). Exposure to Hat results in a surface saturated with OH groups (parts A and B of Figure 9). This reaction is quick. After only 1 L, a saturation coverage corresponding to about one OH groups per surface unit cell is reached. The fact that surface hydroxyls remain stable after finishing the exposure and can only be removed at elevated temperatures as observed by TDS (Figure 9C) means that reduction during exposure does not simply proceed by recombination of adsorbed hydroxyls. We suggest that the occasional formation of initial O vacancies by H2O desorption occurs by a direct interaction of impinging Hat with a surface hydroxyl (Figure 9D), possibly taking advantage of the large amount of energy liberated in the reaction with atomic Hat. If this minority reaction channel thus really follows an Eley-Rideal mechanism needs further evidence. The recombinative H2O formation from two OH groups at elevated temperature (Figure 9C) corresponds to a Langmuir-Hinshelwood mechanism. On a thin FeO film, we have observed that the formation of H2O during exposure to Hat at RT follows an autocatalytic mechanism. We proposed that oxygen vacancies formed initially during reduction destabilize surface hydroxyls in their neighborhood so that they can recombine and form H2O which desorbs. This produces further vacancies thus causing acceleration.28 Since the FeO film was only one Fe-O bilayer thick, the reduction stops after exposure to about 3 L Hat when a final composition of the film corresponding to FeO0.75 is reached. On a thicker Fe3O4 film, the production of H2O and H2 during exposure to Hat was observed to occur in a sequence of irregular bursts, which we interpreted as due to initiation of reduction at defect sites such as oxygen vacancies, possibly also formed statistically at low rate by reaction with Hat. Since the bulk of the thick Fe3O4 film represented a larger reservoir for matter, exchange of Fe or O with the bulk restores the surface and further bursts are possible. Like in the case of Fe2O3 discussed in this paper, the Fe3O4 surface remained hydroxylated after stopping the exposure. The data presented here for Fe2O3 are explainable without the assumption of an autocatalytic reduction mechanism. The removal of 5 H2O molecules per surface unit cell upon exposure to 1000 L Hat may well be explained by direct reaction of surface hydroxyls with impinging Hat, as described above. However,
2204 J. Phys. Chem. C, Vol. 111, No. 5, 2007 we consider it likely that a defect-mediated recombination of hydroxyls such as on FeO and Fe3O4 occurs also on Fe2O3, resulting in an autocatalytic mechanism. Further experiments are necessary to check this. 5. Conclusion We have investigated the interaction of atomic hydrogen with a 3-4-nm thick R-Fe2O3(0001) hematite film with biphase surface structure consisting of periodically arranged FeO(111) and R-Fe2O3(0001) domains. Surface hydroxylation is a fast process and results in a saturation coverage corresponding to about one OH per surface unit cell. Upon heating, the hydroxyls recombine to H2O or H2, which desorb. Parallel to hydroxylation, the oxide is also reduced during exposure at room temperature in a slower process. Under the used conditions, reduction ends when the total film has reached the oxidation state of magnetite Fe3O4. Even at RT, mobility of oxygen or iron is thus sufficient. Arguments were given that this reduction follows an Eley-Rideal mechanism with impinging Hat interacting directly with an OH group at the surface, forming H2O which desorbs. The study illustrates that the surface hydrogen formed in catalytic dehydrogenation over hematite (like the styrene synthesis) is responsible for the observed reduction of the catalyst.22 Also the relative stability of the final Fe3O4 agrees with the observation that the catalyst can be stabilized at this oxidation state.22 Acknowledgment. W.H. gratefully acknowledges a scholarship from the Alexander von Humboldt Foundation for his stay at Fritz Haber Institute. This work was supported by National Natural Science Foundation of China (Grant 20503027) and by the “Hundred Talent Program” of Chinese Academy of Sciences. References and Notes (1) Iwasawa Y. 11th International Concress on Catalysis-40th AnniVersary; Elsevier: Amsterdam, 1996; p. 21. (2) Henrich, V. E.; Cox, P. A. The surface science of metal surfaces; Cambridge University Press: 1994. (3) Goodman, D. W. Chem. ReV. 1995, 95, 523-536. (4) Barteau, M. A. Chem. ReV. 1996, 96, 1413-1430. (5) The chemical physics of solid surfaces and heterogeneous catalysis 9: Oxide surfaces; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 2001. (6) Mokwa, W.; Kohl, D.; Heiland, G. Surf. Sci. 1980, 99, 202-212.
Huang et al. (7) Genger, T.; Hinrichsen, O.; Muhler, M. Catal. Lett. 1999, 59, 137141. (8) Solomon, E. I.; Jones, P. M.; May, J. A. Chem. ReV. 1993, 93, 2623-2644. (9) Henry, C. Surf. Sci. Rep. 1998, 31, 231-325. (10) Becker, Th.; Ho¨vel, St.; Kunat, M.; Boas, Ch.; Burghaus, U.; Wo¨ll, C. Surf. Sci. 2001, 486, L502-L506. (11) Kunat, M.; Burghaus, U.; Wo¨ll, C. Phys. Chem. Chem. Phys. 2003, 5, 4962-4967. (12) Wang, Y.; Meyer, B.; Yin, X.; Kunat, M.; Langenberg, D.; Traeger, F.; Birkner, A.; Wo¨ll, C. Phys. ReV. Lett. 2005, 95, 266104. (13) Yin, X. L.; Birkner, A.; Ha¨nel, K.; Lo¨ber, T.; Ko¨hler, U.; Wo¨ll, C. Phys. Chem. Chem. Phys. 2006, 8, 1477-1481. (14) Kochloefl K. Handbook of Heterogeneous Catalysis; VCH: Weinheim, 1997; Chapter 4.3.2, pp 2151-2159. (15) Shaikhutdinov, S. K.; Joseph, Y.; Kuhrs, C.; Ranke, W.; Weiss, W. Faraday Discuss. 1999, 114, 363-380. (16) Weiss, W.; Schlo¨gl, R. Top. Catal. 2000, 13, 75-90. (17) Kuhrs, C.; Arita, Y.; Weiss, W.; Ranke, W.; Schlo¨gl, R. Top. Catal. 2001, 14, 111-123. (18) Kuhrs, C.; Swoboda, M.; Weiss, W. Top. Catal. 2001, 15, 13-18. (19) Ketteler, G.; Ranke, W.; Schlo¨gl, R. J. Catal. 2002, 212, 104111. (20) Weiss, W.; Ranke, W. Prog. Surf. Sci. 2002, 70, 1-151. (21) Shekhah, O.; Ranke, W.; Schu¨le, A.; Kolios, G.; Schlo¨gl, R. Angew. Chem., Int. Ed. 2003, 42, 5760-5763. (22) Shekhah, O.; Ranke, W.; Schlo¨gl, R. J. Catal. 2004, 225, 56-68. (23) Ranke, W.; Shekhah, O. Recent Research DeVelopment in Surface Science 2004, 1, 75-99. (24) Schu¨le, A.; Shekhah, O.; Ranke, W.; Schlo¨gl, R.; Kolios, G. J. Catal. 2005, 231, 172-180. (25) Weiss, W.; Zscherpel, D.; Schlo¨gl, R. Catal. Lett. 1998, 52, 215220. (26) Muhler, M.; Schlo¨gl, R.; Ertl, G. J. Catal. 1992, 138, 413-444. (27) Huang, W.; Ranke, W.; Schlo¨gl, R. J. Phys. Chem. B 2005, 109, 9202-9204. (28) Huang, W.; Ranke, W. Surf. Sci. 2006, 600, 793-802. (29) Condon, N. G.; Leibsle, F. M.; Lennie, A. R.; Murray, P. W.; Vaughan, D. J.; Thornton, G. Phys. ReV. Lett. 1995, 75, 1961-1964. (30) Bischler, U.; Bertel, E. J. Vac. Sci. Technol. A 1993, 11, 458460. (31) Ketteler, G.; Weiss, W.; Ranke, W.; Schlo¨gl, R. Phys. Chem. Chem. Phys. 2001, 3, 1114-1122. (32) Lad, R. J.; Henrich, V. E. Surf. Sci. 1988, 193, 81-93. (33) SchedelNiedrig, T.; Weiss, W.; Schlo¨gl, R. Phys. ReV. B 1995, 52, 17449-17460. (34) Joseph, Y.; Ketteler, G.; Kuhrs, C.; Ranke, W.; Weiss, W.; Schlo¨gl, R. Phys. Chem. Chem. Phys. 2001, 3, 4141-4153. (35) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564-5174. (36) Aronniemi, M.; Sainio, J.; Lahtinen, J. Surf. Sci. 2005, 578, 108123. (37) Berdunov, N.; Murphy, S.; Mariotto, G.; Shvets, I. V. Phys. ReV. B 2004, 70, 085404.
